For producing semiconductors, silicon wafers are coated with a light-sensitive lacquer, which is referred to as a resist. The structure to be provided on the wafer is created on a mask (photomask or reticle) and projected as an image into the light-sensitive layer of resist by exposure by use of a lithographic scanner. Microscopes are used to check masks for defects. These microscopes are designed in such a way that the aerial image recorded corresponds with great accuracy to the image that is generated on the wafer by the scanner. These microscopes are also referred to as mask inspection microscopes. The use of mask inspection microscopes allows masks to be checked (mask review) without it being necessary to carry out exposure of a wafer for checking a mask.
A mask inspection microscope is used to record the aerial image of a region of a mask. To investigate the behavior of mask structures, the aerial images of a mask may be simulated on the basis of the prescribed structure. Corresponding computer programs for the simulation of aerial images are known.
In order to obtain from the aerial image of a mask the expected structure that is obtained on the wafer, the contrast-enhancing effect of the resist, the light-sensitive layer, must be simulated. This is achieved in an easy way by the fixing of an intensity threshold value (also referred to hereinafter simply as the threshold value) and application of this threshold value to the aerial image. The threshold value may be specified as a proportion of the maximum intensity value of an aerial image. This method is referred to hereinafter as the threshold value method.
The threshold value discussed here is, to a good approximation, a measure of the dose. If the mask is used in a scanner for exposing a wafer, the illumination intensity in the exposure, the dose, is adapted in such a way that the structure depicted as an image in the resist corresponds to the prescribed dimensions within the tolerance limits.
The operation that is performed when exposing the resist and its subsequent development can be simulated with great accuracy. In addition to the threshold value or the dose, a number of parameters that describe the resist used are prescribed for this purpose. These parameters are then taken into consideration in the determination of the wafer structure from the aerial image. Such computer programs are known and are referred to as resist simulators, the method as resist simulation. Such a method is disclosed for example in the German laid-open specification DE19757696.
For the determination of a wafer structure from an aerial image, a threshold value or a dose must be prescribed. Until now, for this a threshold value has been chosen arbitrarily. When choosing this threshold value, it is not known whether it will lead to the desired structure.